The present invention relates generally to mechanical strain gages and more specifically to a design for an optical strain gage useable with fiber optic systems.
This invention is intended to provide a sensitive cheap and simple method of monitoring bending strain that is compatible with optical fiber data collection and recording systems.
Strains are typically measured with resistance gages and the gage factor is the ratio of the resistance change (as the gage is strained) to the initial (unstrained) resistance divided by the change in strain. The gage factors for typical resistance strain gages are in the range of 5–10. The optical strain gage technique that has shown the most sensitivity is also highly sensitive to thermal expansion, which can be confused with strain sensitivity unless suitable reference/compensation channels are provided. The hollow tube strain gages described here do not require temperature correction since the length change as the tube is strained length is insignificant.
Optical strain gages, and in particular an optical strain gage useful as a pressure transducer, are known in the art. U.S. Pat. No. 5,101,664, Optical Pressure Transducer, issued on Apr. 7, 1992, to Hockaday, et al shows a micromachine silicon pressure transducer which employs a single vibrating bridge and pressure responsive diaphragm formed from a single silicone wafer. The initial wafer is micromachined by a combination of etching and laser techniques so as to result in a single strand of silicon supported at each end by blocks or other supports also cut from the initial silicon wafer. Other pertinent patented examples are disclosed in the following U.S. patents, the disclosures of which are incorporated herein by reference:
U.S. Pat. No. 5,812,251, Sep. 22, 1998, Electro-optic strain gages and transducer, Manesh, Ali.
U.S. Pat. No. 5,447,075, Sep. 5, 1995, Self-exciting optical strain gage, Hockaday, Bruce.
U.S. Pat. No. 5,309,772, May 10, 1994, Self-exciting optical strain gage, Hockaday, Bruce.
U.S. Pat. No. 5,038,622, Aug. 13, 1991, Strain gage assembly for measuring excessive tensile strain of a flexible elongated member, such as a steel structural cable, or an electrical transmission cable, or a class fiber optic communication cable, which may slightly twist under tensile strain, Tijmann, Willem B.
U.S. Pat. No. 4,815,855, Mar. 28, 1989, Interferometric load sensor and strain gage, Dixon, William P.